1. Field of the Invention
The invention pertains to angle of signal arrival measurements and more specifically to the measurement of the vertical angle of the signal arriving from a source not necessarily in the horizontal plane of the receiver.
2. Description of the Prior Art
Interferometric methods for measuring arrival angles of received signals have long been in use in the sonar and radar art. In these systems, the time difference of arrival at receivers positioned a predetermined distance apart is determined by correlation techniques, if the signals are continuous, or by timing techniques, if the signals are pulses. This difference in time of arrival and the separation distance is utilized to determine the angle of arrival of the signal with an accuracy that is a function of the receiver separation, improving as the separation increases. The measured angle is in the plane of signal propagation defined by the two receivers and the source. In many applications, however, the horizontal plane source angle i.e. azimuth or bearing is required rather than the angle in the signal propagation plane provided by the interferometer. A conversion from the signal propagation angle to the horizontal plane angle may readily be realized with knowledge of the vertical angle of arrival of the signal. Vertical angle of arrival is also useful for other applications, such as establishing the relative altitude or depth of the signal source. Prior art methods of measuring vertical angle are of limited accuracy and resolution capability.
Vertical angle measurements have been made in the prior art with two beams having peaks offset at equal and opposite angles from a reference angle to establish equal amplitude responses for signals incident from the reference direction. In a sonar system, an acoustic signal arriving from the reference angle direction induces electrical signals in the beam transducers of equal magnitude establishing a zero signal difference therebetween. Acoustic signals arriving from angular directions other than the reference angle induce electrical signals that differ from zero, having a magnitude as a function of the angle off the reference angle and a polarity which is determined by whether the arrival angle is less than or greater than the reference angle. The accuracy of these systems is generally poor, being a function of the relative beam shapes of the transducers and the interpolation.
Greater accuracy than that obtainable with amplitude interpolation systems may be achieved with the utilization of a vertically split array to effectively establish two transducer arrays with a physical separation therebetween. These systems determine the time difference of arrival of a signal incident to the dual array. This time difference is a function of the angle from the perpendicular to the array surface and does not depend on the array beam shape, being only a function of the angle of arrival and the dual array separation. Array separation, however, is generally small requiring that the difference between two nearly equal times of arrival be determined, thus limiting the accuracy of the system. Additionally, the small separation between the dual arrays causes the noise at the output terminals of each array to be correlated, adversely affecting the signal-to-noise ratio of the system and concomitantly the angle determination accuracy. Further, the dual arrays overlap in the azimuthal plane and thereby cannot resolve targets within the azimuthal beam width of the arrays. This limitation of bearing resolution causes the dual array to respond to the centroid of multiple targets within the aximuthal beam width.